An Integrated Services Digital Network (ISDN) provides voice and data communications between an originating endpoint, through a combination of one or more private branch exchanges (PBX's) and/or telephone company central and regional offices, to a destination endpoint. Evolving ISDN standards being developed by the International Telephone and Telegraph Consultative Committee (CCITT) define signaling (call setup) and data transfer protocols for each interface (reference point) in this link.
The `S` interface is the point at which computers and terminals attach to the ISDN. The `S` interface is supported on the Network Terminating (NT) equipment side by the AMD DEC chip (79C31) and on the Terminal Equipment (TE) side by the AMD DSC chip (79C30). In large installations, the NT equipment may be a PBX; in smaller installations (e.g., homes), a single TE may be connected to a single telephone line NT. Terminals which communicate using pre-ISDN standards (e.g., RS232) attached at a reference point `R` and a Terminal Adaptor (TA) is used for protocol conversion/rate adaption.
In the initial ISDN scheme, signaling (D-channel) and voice/data (B-change) bit streams are time division multiplexed on the same `S` interface telephone line. (The voice phone analogy for what takes place on the D-channel is the dial-ringing-answer sequence; the analogy for the B-channel is the conversation that takes place once the call is answered.)
Data rates on the ISDN will be substantially higher than those possible today in the public telephone network. In order to provide reliable error-controlled data transmission on the ISDN, members of the Bit-Oriented Protocol (BOP) family of protocols (e.g., LAPD, LAPB) will be used on both D- and B-channels.
Two major `S` interface types, Primary Rate Interface and Basic Rate Interface, are being defined in initial ISDN specifications.
The Primary Rate Interface consists of 23 64 Kbps and one 64 Kbps D-channel used for call/teardown for all of the B-channels. It is anticipated that this 23B+D interface will be used to connect relatively large minicomputers, mainframes, and internetwork gateways to the ISDN.
The Basic Rate Interface consists of two 64 Kbps B-channels and one 16 KBPS D-channel used for call setup/teardown for both of the B-channels. This 2B+D interface will be used to connect "dumb" terminals, integrated voice/data workstations (IVDW's) and personal computers (PC's) to the ISDN.
Each of the B-channels in a 2B+D interface may be used for either digitized voice or data calls. Data calls may be made to either 2B+D or 23B+D endpoints. A given data call may be setup for file transfer between computers or for an interactive session between a human user and a computer.